This invention relates in general to signal processing, and, more specifically, to a method of transmitting surround sound through a noisy medium, such as a coaxial cable.
Currently available devices, commonly referred to as modulators, for transmitting sound according to Multichannel Television Sound (MTS) standard lack the ability to transmit surround sound information due to signal attenuation and noise. According to the MTS standard, the L-R signal, which represents the difference between left (L) and right (R) channel signals, contains the stereo information as well as the surround sound information. The surround sound information is carried at higher frequencies than the stereo information. The monaural signal, which is the L+R signal, is transmitted as a frequency modulated carrier, according to MTS standards. On the other hand, the L-R signal is transmitted as an amplitude modulated (AM) suppressed carrier at twice the horizontal scanning frequency of a television according to MTS standards. In order to receive and detect the L-R signal accurately, attenuation of an AM signal must be minimized. Most modulators currently available can accurately transmit information at the lower frequencies, but have excessive roll-off at the higher frequencies, which prevents transmission of surround sound information. The greater the roll-off at higher frequencies, measured in dBu, the greater the attenuation of the signal at the higher frequencies, and therefore, the greater the attenuation of the surround sound information. At some point in the frequency spectrum the roll-off is so great that the surround sound information can not be distinguished from the noise, which results in loss of the surround sound information. Consequently, only the stereo information is transmitted and detected while the surround sound information is lost. For example, some systems have a 15 dB roll-off at about 8 kHz, which means that more than half of the signal strength is lost due to attenuation, which results in loss of the surround sound information carried at the higher frequencies of the L-R signal.
Therefore, what is needed is an apparatus that transmits the stereo and the surround sound information of the L-R signal without excessive attenuation at the higher frequencies in order to allow transmission of surround sound through a noisy medium.